The Abundance of X-Shaped Radio Sources: Implications for the Gravitational Wave Background

TL;DR

This study investigates the prevalence of X-shaped radio sources as indicators of supermassive black hole mergers and finds their abundance is lower than previously thought, implying a smaller gravitational wave background.

Contribution

It provides a revised estimate of genuine X-shaped radio sources, reducing the expected gravitational wave background from SMBH coalescences based on radio observations.

Findings

Fewer than 21% of candidates are genuine X-shaped sources.

Less than 1.3% of extended radio sources are due to axis reorientations.

The gravitational wave background from SMBH mergers may be significantly smaller.

Abstract

Coalescence of super massive black holes (SMBH's) in galaxy mergers is potentially the dominant contributor to the low frequency gravitational wave background (GWB). IIt was proposed by Merritt and Ekers (2002) that X-shaped radio galaxies are signposts of such coalescences, and that their abundance might be used to predict the magnitude of the gravitational wave background. In Roberts et al. (2015) we present radio images of all 52 X-shaped radio source candidates out of the sample of 100 selected by Cheung (2007) for which archival VLA data were available. These images indicate that at most 21% of the candidates might be genuine X-shaped radio sources that were formed by a restarting of beams in a new direction following a major merger. This suggests that fewer than 1.3% of extended radio sources appear to be candidates for genuine axis reorientations ("spin flips"), much smaller than the 7% suggested by Leahy and Parma (1992). Thus the associated gravitational wave background may be substantially smaller than previous estimates. These results can be used to normalize detailed calculations of the SMBH coalescence rate and the GWB.